Over voltage protection circuit

ABSTRACT

An over voltage protection circuit for preventing excessive high voltages which exceed the X-ray isodose limits for human beings when the high voltage primary winding, the tuning coil, or the high voltage primary driving winding opens. The B+ voltage for the driving oscillator or horizontal driver is derived at a point in the circuit such that when circuit elements fail, the B+ voltage is removed thus turning off the drive to the high voltage circuit.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates in general to protection circuits and in particular to an overvoltage protection circuit for high voltage television circuits.

2. Description of the Prior Art

In television sets the horizontal output transformer has been driven in a manner such that in the event the high voltage primary winding or the tuning coil of a harmonically tuned high voltage transformer open circuits, the high voltage rises to values far above the X-ray isodose limits. Such condition is dangerous to human beings as exposure to excessive high radiation dosage is unsafe.

SUMMARY OF THE INVENTION

The present invention provides a protection circuit such that in the event of failure of circuit elements such as the high voltage primary winding, the tuning coil of a harmonically tuned high voltage transformer, and/or the high voltage primary driving winding will result in the B+ voltage for one of the driving circuits going to zero or to a low value so as to turn off the driving circuit thus eliminating the production of unsafe high voltages which would cause X-ray radiation above the safe limits.

Other objects, features and advantages of the invention will be readily apparent from the following description of certain preferred embodiments thereof, taken in conjunction with the accompanying drawings, although variations and modifications may be effected without departing from the spirit and scope of the novel concepts of the disclosure, and in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of the improved invention.

FIG. 2 illustrates the prior art circuit, and

FIG. 3 is a schematic view of a modification of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 2 is an electrical schematic of a prior art circuit in which a horizontal output transistor T₂ has its emitter electrode connected by diode D₁ to its collector. An inductor L₁₈, and an inductor L₁₇ are connected in series between the emitter and the base. Inductor L₁₇ is the secondary of a transformer and the primary is connected to a suitable driving voltage not shown. A pair of condensers C₇ and C₈ are connected in parallel between the collector and emitter of transistor T₂. The collector of transistor T₂ is coupled through winding L₁₁ to winding L₁₅ which is the high voltage primary and to winding L₁₄ which is connected to the end of L₁₅ and which supplies a tripler 13 that provides a high voltage output on terminal 9. Resistors R₄, R₅, and R₆ are connected between one terminal of the tripler 13 and ground and a wiper contact 15 engages resistor R₅ and is connected to focus control through a resistor R.sub. 7. A tuning coil L₁₆ is connected in parallel with a capacitor C₁₀ and has one side connected to L₁₅ and the other side connected to L₁₂ which has its opposite end connected to the collector of transistor T₂. A condenser C₉ is connected between a junction point between the tuning coil and inductor L₁₂ and ground. A resistor R₃ is connected from the tuning coil L₁₆ to an inductor L₄₁ and has its other side connected to the collector of transistor T₂ to supply B+ voltage. A capacitor C₆ has one side connected to ground and the other side connected to parallel connected yokes L₈ and L₉ which have their other side connected to one end of winding L₁₀ which has its other side connected to winding L₁₁. A convergence output is provided by winding L₁₃ connected between ground and an output convergent point.

In the circuit shown, the voltage at the junction point between windings L₁₄ and L₁₅ might be 1140 volts, and the voltage at the junction between the yokes L₈ and L₉ and the winding L₁₀ might be 1280 volts. If the tuning coil winding L₁₆ opens or if the high voltage primary winding L₁₅ opens, a B+ will still be derived from the resistor R₃ and will keep transistor T₂ turned on; however, the high voltage output of the tripler will go far above the X-ray isodose limits which produces a dangerous hazard to humans and animals under this condition.

The present invention eliminates this hazard with the circuits illustrated in FIGS. 1 and 3 which are two different modifications and utilize the features wherein in the event a faulty high voltage primary or tuning coil exists, the turn-on B+ voltage to the horizontal driver will be turned off such that the dangerous high voltage does not cause X-ray radiation so as to endanger life.

FIG. 1 illustrates the embodiment of the invention wherein a horizontal output transistor T₁ receives a driving signal on its base and has its emitter connected to ground. Its collector is connected to drive the horizontal output transformer comprising windings L₂, L₃, L₄, and L₅. Tripler 13 is connected to the end of winding L₂ and provides high voltage output at its output terminal 9. The yoke comprises windings L₆ and L₇ connected in parallel between the winding L₄ and a capacitor C₄ which has its opposite side connected to ground. One end of the winding L₅ is connected to ground through a capacitor C₅ and the B+ voltage is applied to terminal 12 between capacitor C₅ and winding L₅. A tuning circuit comprises inductor L₁ and capacitor C₂ connected in parallel between one end of winding L₃ and a capacitor C₃ which is connected between ground and one end of winding L₁ and this junction point supplies horizontal driver B+ voltage through a resistor R₁ and a winding L₄₂.

If the high voltage primary winding L₃ opens or the inductor tuning coil L₁ opens, the B+ voltage for the horizontal driver T₄ goes to a value which will turn off transistor T₄ thus assuring that in the event of the failure of the high voltage winding or the tuning coil L₁, dangerous X-ray radiation does not occur.

A modification of the invention is illustrated in FIG. 3, wherein oscillator, driver 21 receives its supply voltage through L₄₈, R₁₀ and primary driving winding L₂₀ which has its other side connected to the coil L₃₃ and condenser C₁₅ connected in parallel. The other side of the coil L₃₃ is connected to the high voltage primary L₃₄ which has its other end connected to the tripler 13 which supplies high voltage output at terminal 9. The midpoint of the high voltage primary L₃₄ is connected to a collector of transistor T₃ which is the horizontal output transistor and which has its emitter connected to the parallel connected resistor R₂₅ and capacitor C₁₃ with the opposite end connected to a winding L₂₄ which has its other end connected to the base of transistor T₃ through the inductor L₂₃. A diode D₃ is connected between the emitter and collector of transistor T₃. A condenser C₁₂ is connected in parallel with the diode D₃. Yoke windings L₂₆ and L₂₇ are connected between the collector of transistor T₃ and a condenser C₁₆ which has its opposite side connected to the emitter of transistor T₃. A winding L₅₆ is connected between the emitter of transistor T₃ and ground. A source power B+ point 18 is connected to a suitable B+ source and is connected to one end of winding L₂₁ which has its other end connected to winding L₂₂ which is connected to the collector of transistor T₃ and to the A point of winding L₃₄. Windings of the yokes L₃₀ and L₃₁ are connected between the capacitor C₁₆ and the connection point between windings L₂₁ and L₂₂.

A winding L₄₈ has one end connected to the oscillator, driver 21, and its other side connected to ground through a capacitor C₂₁. A resistor R₁₀ is connected from winding L₄₈ to the oscillator, driver 21 to supply B+ voltage.

The oscillator, driver 21 receives current through the high voltage primary driving winding L₂₀, the tuning coil L₃₃, the high voltage winding L₃₄, and if any of these windings open, the B+ source to oscillator, driver 21 is turned off and the oscillator driver is rendered inactive such that high voltage does not cause X-ray radiation which is hazardous to personnel.

It is seen that this invention provides means for assuring that high level X-ray radiation does not occur in the event of failure of components in a high voltage circuit thus assuring the safety of personnel. Although the invention has been described with respect to preferred embodiments, it is not to be so limited as changes and modifications may be made which are within the full intended scope as defined by the appended claims. 

I claim as my invention:
 1. An overload protection circuit for a high voltate television circuit comprising an oscillator driver, a high voltage primary driving winding connected to said oscillator driver, a tuning coil connected in series with said high voltage primary driving winding, a high voltage primary connected in series with said tuning coil and driving winding, a horizontal driver connected to said high voltage primary, a B+ source connected in series with said primary driving winding, said tuning coil, and said high voltage primary and connected to said oscillator driver to provide supply voltage therefor.
 2. An overload protection circuit for a high voltage circuit for a television receiver comprising, a horizontal driver transistor, a horizontal output transistor, a high voltage transformer connected to said horizontal output transistor, a tuning coil, a B+ voltage source, a high voltage primary driving winding connected in series with said tuning coil and said high voltage transformer, said B+ source being removed from said horizontal driver transistor if said driving winding, said tuning coil or said high voltage transformer open circuit, and conductor means connecting said B+ voltage source to said horizontal driver transistor. 